The present invention relates to disc drives. More particularly, the present invention relates to a data detector in a disc drive wherein the data detector accounts for correlated noise.
A typical disc drive includes one or more magnetic discs mounted for rotation on a hub or spindle. A typical disc drive also includes a transducer supported by a hydrodynamic air bearing which flies above each magnetic disc. The transducer and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the magnetic discs and to store information on the magnetic discs.
An electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations.
Information is typically stored in concentric tracks on the surface of magnetic discs by providing a write signal to the data head to encode flux reversals on the surface of the magnetic disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the data head flies above the magnetic disc, sensing the flux reversals on the magnetic disc, and generating a read signal based on those flux reversals. The read signal is typically conditioned and then decoded by the drive controller to recover data represented by flux reversals stored on the magnetic disc, and consequently represented in the read signal provided by the data head.
A typical read channel includes the data head, preconditioning logic (such as preamplification circuitry and filtering circuitry) a data detector and recovery circuit, and error detection and correction circuitry. The read channel can be implemented either as discrete circuitry, or in a drive controller associated with the disc drive.
A Viterbi detector has been investigated in the past for use as a data detector in a disc drive read channel. A Viterbi detector acts as a maximum-likelihood sequence estimator when the input to the detector consists of a signal plus additive white, Gaussian noise, and when a typical branch metric (the square of the error in the signal provided to the detector) is used. However, in an actual disc drive channel, the noise from the media is generally colored. In addition, in order to practically implement a Viterbi detector in a disc drive, a filter or equalizer must be used in order to obtain a simplified partial-response polynomial target. Thus, the noise at the input to the detector is further colored by the filter or equalizer which is required to simplify the target response. The filter or equalizer correlates or combines noise in the system by combining noise components from several different instants in time.
Conventional Viterbi detectors are utilized with branch metrics that do not take into account the correlation of noise in the filter or equalizer. Thus, if the noise is colored, as is almost always the case, the Viterbi detector is suboptimum. For example, if the target response is a simplified EPR4 target, the Viterbi detector gives up in excess of 2 dB in the signal-to-noise ratio over a maximum likelihood sequence estimator. Thus, to date, the Viterbi detector has been suboptimimum for use in magnetic disc drives.